Wireless communications continue to gain in popularity, but wireless communications are constrained due to a lack of available, interference free spectrum that may be used for reliable communications within a geographic area. Moreover, wireless systems often have power, frequency and timing regulations or constraints. Power restrictions are often imposed by regulatory authorities. Unlicensed WiFi devices, for example, are governed by FCC PARIS 5 restrictions on power that limit the devices to a few hundred mW indoors. Frequency accuracy requirements are often driven by standards organizations, vendors and operators to ensure good interoperability between network infrastructure and user devices. Cellular systems, which operate at higher power in licensed spectrum, have stringent requirements for frequency accuracy of transmitters, typically under 100 parts per billion.
Most cellular systems deployed today operate in a frequency-division duplex (FDD) configuration where the transmitter and receiver operate at different carrier frequencies. A forward communication link from base stations to wireless terminals is carried on a different frequency from a reverse link from the wireless terminals to the base stations. That means for every band that a phone supports, it actually uses two frequency ranges, which are known as paired frequency bands. FDD communications require very little coordination between base stations, but do require that paired spectrum be available.
Wireless communication in time-division duplex (TDD) configuration is also feasible. TDD uses one single range of frequencies in a frequency band, but that band is segmented to support transmit and receive signals in a single frequency range. Specifically, TDD applies time-division multiplexing to separate outward and return signals. The forward and reverse links simply alternate in time. One of the advantages of TDD communication is that paired spectrum is not required and two-way communication can be achieved in a single slice of spectrum. While only a very small number of TDD networks are operational in the world today, the utility and value of TDD spectrum is growing rapidly across the world due to the exhaustion of easily pairable bands to support FDD operation.